The present invention relates to integrated circuit fabrication and, more specifically, to optical elements, such as extreme ultraviolet mirrors and masks, used in lithography and methods for manufacturing an optical element, such as an extreme ultraviolet mirror or mask.
Semiconductor chips may be manufactured using processes that involve lithography. Immersion lithography is a lithography technique that uses ultraviolet light with a wavelength equal to 193 nanometers; and the wafer is immersed in water for improved resolution. Extreme ultraviolet (EUV) lithography is a technique that has been explored to extend lithographic technology beyond the optical limits of immersion lithography to pattern small critical dimension features. Extreme ultraviolet lithography employs radiation in the extreme ultraviolet (EUV) region of the electromagnetic spectrum that has a shorter wavelength, e.g., 13.5 nanometers, than in immersion lithography.
A EUV scanner directs EUV radiation from a EUV source to expose a resist layer on a wafer. The EUV scanner relies on optical elements, e.g., mirrors and a mask, that are reflective rather than the refractive optical elements, i.e., lenses, traditionally found in an immersion lithography system. For example, a EUV scanner may include multiple condenser mirrors, multiple projection mirrors, and a mask as optical elements. The optical elements of a EUV scanner include a multi-layer structure with periods consisting of silicon and molybdenum bi-layers, and reflection from the multi-layer structure of each optical element occurs by the mechanism of Bragg interference. The multi-layer structure of each optical element absorbs a fraction (e.g., 30 percent) of the incident EUV radiation and reflects the remainder of the incident EUV radiation, which is a disadvantage due to the number of mirrors that are present in the EUV scanner.